Method for removing corrosive sulfur compounds from a transformer oil

ABSTRACT

A method removes corrosive sulfur compounds from transformer oil. By adding a mixture of rare earths containing aluminum oxide and aluminum silicate to the transformer oil, and enriching the same with an aqueous solution of soluble metal salts, the corrosive sulfur compounds in the transformer oil are neutralized with defined heating and cooling phases. Advantageously, no additional chemical components, such as passivators, are added to the transformer oil. When using a tank for receiving the mixture of the rare earths containing aluminum oxide and aluminum silicate, the reaction can run in the tank. Any aging products that may be present, and the bonded corrosive sulfur compounds are effectively retained within the tank by a filter system, and can be disposed of with the tank.

The invention relates to a method for removing corrosive sulfurcompounds from a transformer oil.

Transformers frequently use as insulation and cooling media transformeroils which, due to their long-term chemical characteristics, have formany years been used for operating transformers. One problem associatedwith using transformer oils is, however, the presence of natural oradded sulfur compounds which contribute to the oxidation stability ofthe oil itself, in particular in the case of uninhibited transformeroils. Conductive copper sulfide compounds are consequently formed whichare preferentially deposited in the paper insulation and impair itsinsulating properties. This phenomenon is promoted in particular atelevated operating and ambient temperatures.

When unlacquered, paper-insulated copper conductors are used within atransformer and under conditions of limited oxygen content, for examplewhen a transformer is operated with exclusion of air, transformer oilscomprising corrosive sulfur-containing constituents form layers ofcopper sulfate on the paper insulation. Starting from the copperconductor, copper sulfide layers form within the paper layerssurrounding the copper conductor. As a result, the insulation propertiesof the paper insulation are sometimes durably impaired, such thatpartial discharges and voltage flashovers may occur between the livecopper conductors due to the reduced insulation properties of the paperinsulation.

These corrosive sulfur compounds, in particular mercaptans anddisulfides, form above all in transformers, chokes or passages underspecific operating and temperature conditions and reduce the insulationproperties of the paper insulation to a considerable extent; sometimesdown to just 20 percent of the original electric strength of the paperinsulation.

The attempt has accordingly been made in the prior art to suppress thereaction of the corrosive sulfur compounds within the transformer oilswith the copper conductor and simultaneously to improve oxidationresistance by “passivating” the transformer oils, in particular by meansof metal passivators comprising benzotriazole-based compounds. A problemin this case is in particular that the metal passivator may be consumedduring the ongoing operation of the transformer and the quantity ofpassivator available must thus be permanently monitored. Moreover, theextent to which long-term passivation modifies the properties of thetransformer oils is as yet unknown.

WO 2005/117031 A2, for example, accordingly describes a method and adevice for adding a passivator to a conductor. The above-stated patentapplication proposes winding the passivator directly around theconductor and then sheathing it with a further layer of an electricalinsulator and so providing overall electrical insulation for theconductor with the passivator layer.

WO 2007/096709 A2 moreover describes a method for permanently removingcorrosive components from a transformer oil. The above-stated patentapplication proposes removing the transformer oil from a transformertank and, after heating and addition of an acid-containing liquid,bringing it into contact by means of a sulfide free-radical scavengerand then filtering it. After filtration, the transformer oil purified inthis manner is reintroduced into the transformer tank.

The same applies to WO 2007/144696 A2 as a method for deactivatingcorrosive sulfur in transformer oils. According to the invention, theabove-stated patent application proposes adding a sulfide-formingchemical component to the transformer oil comprising corrosive sulfurcompounds, such that said chemical component reacts with the sulfurcompound and the corrosive sulfur compounds are thus removed from thetransformer oil.

DE 10 2005 006 271 A1 moreover describes a method for purifyingtransformer oil, the transformer oil initially being subjected to apretreatment by filtration, before it is passed through a packing of aninert inorganic support coated with a reactive metal. The transformeroil is then filtered through a bleaching earth bed and then returned tothe transformer.

The object of the present invention is accordingly to avoid thedisadvantages in the prior art and to provide a method for removingcorrosive sulfur compounds from a transformer oil which easy to handleand ensures virtually complete removal of corrosive sulfur compoundsfrom the transformer oil.

Said object is achieved by the features of the method as claimed inclaim 1. According to the invention, a method is proposed for removingcorrosive sulfur compounds from a transformer oil, in which, withaddition of a mixture of rare earths containing aluminum oxide/aluminumsilicate to the transformer oil, said transformer oil enriched in thismanner is heated to up to 300 degrees Celsius and then, with enrichmentwith an aqueous solution of soluble metal salts, is cooled.

The transformer oil enriched with the rare earth mixture containingaluminum oxide/aluminum silicate is then once more heated to up to 200degrees Celsius for at least two hours and then cooled to roomtemperature.

Heating of the mixture of rare earths containing aluminum oxide/aluminumsilicate activates the adsorption centers of the matrix by removingwater fractions. The heavy metal salts present in the mixture of therare earths containing aluminum oxide/aluminum silicate are thereafterdispersed in a little water and the mixture is slowly heated. This givesrise to heavy metal oxides which are insoluble and firmly bound to thefuller's earth matrix of the mixture of rare earths containing aluminumoxide/aluminum silicate.

In this manner, the adsorbent is prepared. The solution presented hereis based on removing the reactive corrosive sulfur compounds present inthe transformer oil by using a mixture of inorganic adsorbents with awide range of applications. It mainly comprises a mixture of rare earthscontaining aluminum oxide/aluminum silicate and are optionally enrichedwith silver, copper, zinc and/or iron in metallic or oxide form. Inparticular, the metal oxides formed by means of the rare earth mixturecontaining aluminum oxide/aluminum silicate bind the corrosive sulfurcompounds and may be collected at a suitable point and removed from thetransformer oil, optionally together with simultaneous removal of oilageing products.

The advantage of this method is that no additional foreign substances,such as for example passivators, are added to the transformer oil.Ageing products and corrosive sulfur compounds are simultaneouslyeliminated from the transformer oil. Oxidation capacity is consequentlyincreased and the fraction of corrosive sulfur compounds within thetransformer oil is greatly reduced, so durably increasing the servicelife of the transformer.

It is considered advantageous according to the present method for theratio between the fraction comprising aluminum oxide and the fraction ofaluminum silicate in the rare earth mixture containing aluminumoxide/aluminum silicate to be in a ratio of 20:80 to 80:20, preferablyof 50:50. The catalytic action of the rare earth mixture containingaluminum oxide/aluminum silicate is best ensured within the preferredratio range of the fractions.

A bulk density of 50 to 80 g/l is advantageously used in order toprovide the greatest possible surface area of the rare earth mixturecontaining aluminum oxide/aluminum silicate. In this bulk density range,an in particular granular rare earth mixture containing aluminumoxide/aluminum silicate has an effective surface area for binding thecorrosive sulfur compounds present in the transformer oil. The aqueoussolution advantageously has a solution fraction of up to 40% of inparticular soluble metal salts. Adding copper and silver salts inparticular leads to improved binding of the corrosive sulfur compoundsonto the metal salts present in the aqueous solution. This preciselyprevents the corrosive sulfur compounds present in the transformer oilfrom reacting chemically with the copper conductor. The pH value of therare earth mixture containing aluminum oxide/aluminum silicate isadvantageously 6.5 to 9.0. The metal oxides formed react with thecorrosive sulfur compounds at the highest possible rate of reaction inthe above-stated pH range.

An advantageous development of the method provides that the ratio of therare earth mixture containing aluminum oxide/aluminum silicate totransformer oil, relative to their respective weights, is in a ratio of0.01:100 to 40:100, preferably of 10:100. The highest possible rate ofreaction is ensured in particular at the preferred weight ratio of10:100 of rare earth mixture containing aluminum oxide/aluminum silicateto transformer oil due to their respective concentrations.Advantageously, the rare earth metals of group 3 of the periodic tableof elements including the lanthanoids are a constituent of the rareearth mixture containing aluminum oxide/aluminum silicate. In anadvantageous development of the method, silver, copper, zinc and/or ironare admixed with the rare earth mixture containing aluminumoxide/aluminum silicate. Moreover, silver nitrate to form silver oxidesand/or copper salts to form copper oxides and/or iron oxides is/areadmixed with the rare earth mixture containing aluminum oxide/aluminumsilicate. The metal oxides present in this manner within the rare earthmixture containing aluminum oxide/aluminum silicate are highly reactiveand combine with the corrosive sulfur compounds within the transformeroils and neutralize the corrosive sulfur compounds.

An advantageous development of the method provides that the rare earthmixture containing aluminum oxide/aluminum silicate is arranged in acontainer, in which the container may be fitted on a transformer housingand the transformer oil is passed into the container and purified, andthe sulfides bound in the rare earth mixture containing aluminumoxide/aluminum silicate as reaction products of the corrosive sulfurcompounds remain in the container. Thanks to the reaction of thecorrosive sulfur compounds of the transformer oils within the containerand the accumulation of the bound sulfides in the container, these wasteproducts may be disposed of on removal of the container. At the sametime, any further contamination of the transformer oils with the boundsulfides in the container is ruled out, such that corrosive sulfurcompounds may virtually completely be removed from the transformer oilby the above-stated method.

In the event of complete consumption of the rare earth mixturecontaining aluminum oxide/aluminum silicate, the container isadvantageously removed from the transformer housing. In an advantageousdevelopment of the method, the container comprises an indication of thereactive rare earth mixture containing aluminum oxide/aluminum silicatewhich is present. In the context of servicing, this indication may beused to establish whether sufficient reactive rare earth mixturefractions containing aluminum oxide/aluminum silicate are present andproper performance of the method is ensured.

A filter system is advantageously introduced within the container, thefilter system comprising the rare earth mixture containing aluminumoxide/aluminum silicate, and the transformer oil is introduced into thefilter system. By means of the filter system, the bound sulfides and thetransformer oil end-of-life products may in particular more readily beretained within the filter system and so collected within the container.

An advantageous development of the method provides that the containermay be connected with a purifying device, in which the purifying devicemay be connected with the transformer housing and the transformer oilmay be transferred out of the transformer housing for purification inthe purifying device and thus the corrosive sulfur compounds are removedin the container outside the transformer housing.

Further advantageous developments are revealed by the subclaims.

EXAMPLE

A rare earth mixture containing aluminum oxide/aluminum silicate has abulk density of 600 g/l with a ratio of aluminum oxide to aluminumsilicate of 50:50. The pH value is 7.0. One kilogram of the rare earthmixture containing aluminum oxide/aluminum silicate is activated at 150°C. and, after cooling, treated in portions with 400 ml of a 20% aqueoussolution of soluble salts of silver, copper, zinc or iron. The mixtureis homogenized and heated stepwise to 120° C. within five hours. Thistemperature is maintained for 15 to 20 hours. After cooling, the mixtureis kept in a closed vessel. The ratio relating to the weights of theactive rare earth mixture containing aluminum oxide/aluminum silicate totreated transformer oil is 0.5:100 to 10:100, depending on the state ofageing and corrosiveness of the transformer oil.

1-15. (canceled)
 16. A method for removing corrosive sulfur compoundsfrom transformer oil, which comprises the steps of: adding a mixture ofrare earths containing aluminum oxide/aluminum silicate to thetransformer oil; heating the transformer oil enriched with the rareearth mixture containing the aluminum oxide/aluminum silicate to up to300 degrees Celsius; cooling the transformer oil enriched with the rareearth mixture containing the aluminum oxide/aluminum silicate; enrichingthe transformer oil with an aqueous solution of soluble metal salts; andheating the transformer oil to up to 200 degrees Celsius for at leasttwo hours and subsequent cooling to room temperature.
 17. The methodaccording to claim 16, which further comprises setting a ratio between afraction containing the aluminum oxide and a fraction of the aluminumsilicate in the rare earth mixture containing the aluminumoxide/aluminum silicate to be in a range of 20:80 to 80:20.
 18. Themethod according to claim 16, which further comprises setting a bulkdensity of the mixture of rare earths containing the aluminumoxide/aluminum silicate to be 50 to 800 g/l.
 19. The method according toclaim 16, wherein the aqueous solution has a solution fraction of up to40% of the soluble metal salts.
 20. The method according to claim 16,which further comprises setting a pH value of the mixture of rare earthscontaining the aluminum oxide/aluminum silicate to be 6.5 to 9.0. 21.The method according to claim 16, which further comprises setting aratio of the mixture of rare earths containing the aluminumoxide/aluminum silicate to the transformer oil, relative to theirrespective weights, to be in a range of 0.01:100 to 40:100.
 22. Themethod according to claim 16, which further comprises using rare earthmetals of group 3 of the periodic table of elements and lanthanoids. 23.The method according to claim 16, which further comprises admixing atleast one of silver, copper, zinc or iron with the mixture of rareearths containing the aluminum oxide/aluminum silicate.
 24. The methodaccording to claim 16, which further comprises admixing at least one ofsilver nitrate to form silver oxides, copper salts to form copperoxides, zinc oxides or iron oxides with the mixture of rare earthscontaining the aluminum oxide/aluminum silicate.
 25. The methodaccording to claim 16, which further comprises disposing the mixture ofrare earths containing the aluminum oxide/aluminum silicate in acontainer, and the container is connected with a transformer housing andthe transformer oil is passed into the container and purified, andsulfides bound in the mixture of rare earths containing the aluminumoxide/aluminum silicate remain in the container.
 26. The methodaccording to claim 25, which further comprises in an event of a completeconsumption of the mixture of rare earths containing the aluminumoxide/aluminum silicate, removing the container from the transformerhousing.
 27. The method according to claim 25, wherein the containercontains an indication of the reactive mixture of rare earths containingthe aluminum oxide/aluminum silicate which is still present.
 28. Themethod according to claim 25, wherein the container has a heating unitfor heating the transformer oil enriched with the mixture of rare earthscontaining the aluminum oxide/aluminum silicate.
 29. The methodaccording to claim 25, wherein a filter system within the containercontains the mixture of rare earths containing the aluminumoxide/aluminum silicate and the transformer oil is introduced into thefilter system.
 30. The method according to claim 25, which furthercomprises connecting the container with a purifying device, thepurifying device being connected with the transformer housing and thetransformer oil may be transferred out of the transformer housing forpurification in the purifying device and thus the corrosive sulfurcompounds are removed in the container outside the transformer housing.31. The method according to claim 16, which further comprises setting aratio between a fraction containing the aluminum oxide and a fraction ofthe aluminum silicate in the mixture of rare earths containing thealuminum oxide/aluminum silicate to be 50:50.
 32. The method accordingto claim 16, which further comprises setting a ratio of the mixture ofrare earth containing the aluminum oxide/aluminum silicate totransformer oil, relative to their respective weights, to be 10:100.